Sources and standard wave shapes of over voltages in power system

Transient over voltages in Power system

Disturbances in electric power transmission and distribution systems are frequently caused by two kinds of transient over voltages. These are two sources of transient over voltage, external and internal, i.e., within the power system. Amplitudes of such transient over voltages may greatly exceed the peak values of the normal ac operating voltage. External sources of over voltage The first of this kind of over voltage is lightning over voltage, originated by lightning strokes hitting the phase wires of overhead lines or the bus bars of outdoor substations. The amplitude of lightning (li)  over voltage depends upon the magnitude of impulse current injected into the power system by the lightning strike and the circuit impedence. If the li strikes on a transmission line, the amplitude of the over voltage is equal to ½ (li current x the surge impedance of the line)  which proceeds as traveling waves in either direction from the point of li strike, Fig 27.1. Fig 27.1 Transient over voltage on transmission line Although the maximum known li impulse current injection is of the order of 220 kA, it could be as low as 200 A also. The average value of this current lies between 10 to 15 kA. The surge impedance of transmission lines varies between 250 to 350 Ω. These together determine the magnitude of the transient over voltage due to lightning strike on a transmission line. The first attenuation of such over voltages is often caused by flash over across the nearest insulator strings on the line. Lightning protection systems, surge arresters and the different kinds of losses in the line damp and distort the traveling wave shapes. Therefore lightning over voltages with very different wave shapes may be present within the transmission system. Internal source of over voltage The second kind of transient over voltage is caused by switching operation in the network, known as 'switching impulse over voltage', si. Normally lagging current flows through a line. When a short circuit occurs, very high magnitude of lagging (inductive) current flows. When a circuit breaker operates (opens) to clear such lagging currents, the voltage impressed across it is greater than the rated voltage of the system. This is because of the energy stored in the inductance of the network which is released when the current stops flowing. The instant energy stored varies with time with the periodic oscillation of the alternating current. Hence, the magnitude of the over voltage impressed across the CB electrodes depends not only upon the value of circuit inductance and the magnitude of current but also upon the instant at which the arc is extinguished. The amplitude of si, also known as switching surge is always related to the operating voltage. Its shape is determined by the impedance of the system as well as by the switching conditions. The rate of rise of switching over voltage transient is usually slower than the li over voltage. However, it is known now that the si wave shape can be very dangerous to insulation systems. With the increasing Basic Insulation Levels (BIL), for 800 kV and above rated voltage level, the lighting overvoltage is not of much significance. However, since the si overvoltage magnitude depends upon the level of the system voltage, these are of much more significant. Fig. 27.2 shows the standard shape of a full lightning impulse voltage as well as a sketch for the same voltage wave chopped at its tail, i.e. interrupted by a disruptive discharge.

Fig 27.2   General shape and definitions of lightning impulse voltage. (a) Full wave   (b) Wave chopped at its tail.

Although the wave shape is well defined on this diagram, it should be emphasized that the virtual origin 01 is defined where the line AB cuts the time axis.The specifications permit in general a tolerance upto ±30% for T1 and ±20% for T2. The reason for defining the point A at 30% voltage level is the experience with most of the oscillograms of measured impulse voltages. The impulse voltage wave shapes are defined by the times  T1 / T2. Hence 1/50 µs is an accepted standard li wave shape. It is quite difficult to obtain a smooth slope within the voltage rise as the measuring systems as well as stray capacitances and inductances may cause oscillations. Figure 27.3 illustrates the wave shape of one of the standard switching impulse. Impulse wave shapes of 100/2500, 250/2500 and 500/2500 µs are recommended.  Permissible tolerance in the case of si for Tcr is ±20% and for T2 it is ±60%.

Fig 27.3  General shapes of switching impulse voltages . Tcr: Time to  crest. T2: Virtual time to half value. Td :  Time above 90%.